When a file system is created, data structures are created that contain information about files. Each file is associated with an inode that is identified by an inode number (often referred to as an "i-number" or "inode") in the file system where it resides.
Inodes basically store information of files and folders, such as user and group ownership, access mode (read, write, execute permissions) and type of file. On many types of file systems the number of inodes available is fixed at file system creation, limiting the maximum number of files the file system can hold. A typical fraction of space allocated for inodes in a file system is 1% of total size.
The inode number indexes a table of inodes in a known location on the device; from the inode number, the kernel can access the contents of the inode, including the data pointers, and so the contents of the file.
A file's inode number can be found using the ls -i command, while the ls -l command will retrieve inode information (i.e. the file information).
Some Unix-style file systems such as ReiserFS may avoid having a table of inodes, but must store equivalent data in order to provide equivalent functions. The data may be called stat data, in reference to the stat system call that provides the data to programs.
File names and directory implications:
* Inodes do not contain file names, only file metadata.
* Unix directories are lists of "link" structures, each of which contains one filename and one inode number.
* The kernel must search a directory looking for a particular filename and then convert the filename to the correct corresponding inode number if the name is found.
The kernel's in-memory representation of this data is called struct inode in Linux. Systems derived from BSD use the term vnode, with the v of vnode referring to the kernel's virtual file system layer.